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The Latest Research Items

Hopping Diffusion of Nanoparticles

In a collaborative work, published in Macromolecules, researchers in the Rubinstein Group propose a hopping mechanism for diffusion of large nonsticky nanoparticles subjected to topological constraints in both unentangled and entangled polymer solids, networks and gels, and entangled polymer liquids, melts and solutions. Probe particles with size larger than the mesh size ax of unentangled polymer networks or tube diameter ae of entangled polymer liquids are trapped by the network or entanglement cells. At long time scales, however, these particles can diffuse by overcoming free energy barrier between neighboring confinement cells.

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The terminal particle diffusion coefficient dominated by this hopping diffusion is appreciable for particles with size moderately larger than the network mesh size ax or tube diameter ae. Much larger particles in polymer solids will be permanently trapped by local network cells, whereas they can still move in polymer liquids by waiting for entanglement cells to rearrange on the relaxation time scales of these liquids. Hopping diffusion in entangled polymer liquids and networks has a weaker dependence on particle size than that in unentangled networks as entanglements can slide along chains under polymer deformation. The proposed novel hopping model enables understanding the motion of large nanoparticles in polymeric nanocomposites and the transport of nano drug carriers in complex biological gels such as mucus.


In-Cell Ribosome Structure Resolved

It has been known for decades that the ribosome, the cellular complex that synthesizes proteins, interconverts between "active" and "inactive" conformations. However, the physiological relevance of this widely observed switch remained unclear and unknown.

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Jennifer McGinnis, in the Weeks Lab, led a study, published in PNAS, in which newly developed in-cell SHAPE technologies were used to probe the structure of the ribosome in healthy living cells. In cells, one class of ribosome subunits exists predominantly in the classic "inactive" conformation and disrupting the ability to interconvert between active and inactive conformations compromises protein synthesis. In-cell RNA structure probing thus resolved this 40 year old challenge to reveal that the inactive state regulates ribosome function as a conformational switch.


NO-Releasing Antibacterial Agents

Researchers in the Schoenfisch Group, have published research in Acta Biomaterialia, where they describe how S-Nitrosothiol-modified chitosan oligosaccharides were synthesized by reaction with 2-iminothiolane hydrochloride and 3-acetamido-4,4-dimethylthietan-2-one, followed by thiol nitrosation. The resulting nitric oxide (NO)-releasing chitosan oligosaccharides stored approximately 0.3 micromol NO mg-1 chitosan. Both the chemical structure of the nitrosothiol, that is primary and tertiary, and the use of ascorbic acid as a trigger for NO donor decomposition were used to control the NO-release kinetics. With ascorbic acid, the S-nitrosothiol-modified chitosan oligosaccharides elicited a 4-log reduction in Pseudomonas aeruginosa viability.

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Confocal microscopy indicated that the primary S-nitrosothiol-modified chitosan oligosaccharides associated more with the bacteria relative to the tertiary S-nitrosothiol system. The primary S-nitrosothiol-modified chitosan oligosaccharides elicited minimal toxicity towards L929 mouse fibroblast cells at the concentration necessary for a 4-log reduction in bacterial viability, further demonstrating the potential of S-nitrosothiol-modified chitosan oligosaccharides as NO-release therapeutics.

Multicolor Monitoring

The proteasome, a validated anticancer target, participates in an array of biochemical activities, which range from the proteolysis of defective proteins to antigen presentation. Researchers in the Lawrence Group, report in ACS Chemical Biology, on the preparation of biochemically and photophysically distinct green, red, and far-red real-time sensors designed to simultaneously monitor the proteasome's chymotrypsin-, trypsin-, and caspase-like activities, respectively.

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These sensors were employed to assess the effect of simultaneous multiple active site catalysis on the kinetic properties of the individual subunits. Furthermore, the team found that the catalytic signature of the proteasome varies depending on the source, cell type, and disease state. Trypsin-like activity is more pronounced in yeast than in mammals, whereas chymotrypsin-like activity is the only activity detectable in B-cells, unlike other mammalian cells. Furthermore, chymotrypsin-like activity is more prominent in transformed B cells relative to their counterparts from healthy donors.


Nanowire Solar Cells

Semiconductor nanowires, NWs, often exhibit efficient, broadband light absorption despite their relatively small size. This characteristic originates from the subwavelength dimensions and high refractive indices of the NWs, which cause a light-trapping optical antenna effect. As a result, NWs could enable high-efficiency but low-cost solar cells using small volumes of expensive semiconductor material. Nevertheless, the extent to which the antenna effect can be leveraged in devices will largely determine the economic viability of NW-based solar cells. Published in Nano Letters, researchers in the Cahoon Group, demonstrate a simple, low-cost, and scalable route to dramatically enhance the optical antenna effect in NW photovoltaic devices by coating the wires with conformal dielectric shells

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Scattering and absorption measurements on Si NWs coated with shells of SiNx or SiOx exhibit a broadband enhancement of light absorption by ≈50–200% and light scattering by ≈200–1000%. The increased light–matter interaction leads to a ≈80% increase in short-circuit current density in Si photovoltaic devices under 1 sun illumination. Optical simulations reproduce the experimental results and indicate the dielectric–shell effect to be a general phenomenon for groups IV, II–VI, and III–V semiconductor NWs in both lateral and vertical orientations, providing a simple route to approximately double the efficiency of NW-based solar cells.


3D Game Changer

Additive manufacturing processes such as 3D printing use time-consuming, stepwise layer-by-layer approaches to object fabrication. As published in Science, and becoming that issue's cover story, researchers in the DeSimone Group demonstrate the continuous generation of monolithic polymeric parts up to tens of centimeters in size with feature resolution below 100 micrometers.

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Continuous liquid interface production is achieved with an oxygen-permeable window below the ultraviolet image projection plane, which creates a "dead zone," or persistent liquid interface, where photopolymerization is inhibited between the window and the polymerizing part. By delineating critical control parameters the researchers show that complex solid parts can be drawn out of the resin at rates of hundreds of millimeters per hour. These print speeds allow parts to be produced in minutes instead of hours, and have become "game changing" properties of this technology.